This invention relates to a photosensitive medium for recording a charge latent image and a recording method thereof.
In compliance with an increased demand for a reproduced image with high picture quality and high resolution in recent years, as well known, various systems such as so-called EDTV (Extended Definition TV) system, or HDTV (High Definition TV) system have been proposed for a television system. In order to obtain a reproduced image with high picture quality and high resolution, it is required to provide an image pickup device capable of producing a video signal from which a high picture quality and high resolution image can be recreated. However, for conventional image pickup devices using an image pickup tube, it is difficult to generate such a video signal. The reasons for this are as follows: Since there is a limit to the reduction of the diameter of an electron beam in the pickup tube, high resolution image reproduction by reduction of the diameter of the electron beam cannot be expected. Alternatively, if the target area of the pickup tube is increased, the level of the output signal will be reduced because of the increased output capacity which is proportional to the area of the target. Therefore, high resolution image reproduction by the increase of the target area cannot be realistic. Furthermore, in the case of an image pickup device for a moving picture, since the frequency range of such a video signal reaches several tens to several hundreds MHz for implementation of the high resolution image, the increase of output capacity, i.e. the increase of target area is not preferable.
On the other hand, an increase of pixels or downsizing a pixel of solid state image sensors has difficulties known to the industry.
As stated above, conventional image pickup devices of either a pickup tube or a solid state sensor could not satisfactorily generate such a video signal to provide a reproduced image of high picture quality and high resolution because of the inevitable use of an image sensor for the construction thereof. In order to solve this, the assignee of this application has already proposed an imaging system and a recording system to obtain a high resolution optical image by an image pickup device using a photo-to-photo transducer, and to record such an optical image as a charge image of high resolution onto a charge accumulation layer (or a charge holding layer) by using a photo-to-charge transducer.
The recording system using a photo-to-charge transducer will be explained with reference to FIG. 1.
Throughout the drawings, like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation.
The photosensitive medium 1 for recording a charge latent image shown in FIG. 1 is composed of a glass substrate 2 allowing an electro-magnetic wave to pass therethrough, an electrode 3 also allowing the electro-magnetic wave to pass therethrough and a photo-conductive layer 4 composed of a photo-conductive material, such as .alpha.-Se (amorphous Selenium) and PVK (Poly-N-Vinylcarbazole), the impedance of which is varied accordingly with the intensity of the electro-magnetic wave. These members are laminated in order, thus constituting the photosensitive medium 1. The electro-magnetic wave in this specification includes an electro-magnetic radiation beam such as X-rays, .gamma.-rays, radio wave or light.
A recording medium 5 is placed to face the photosensitive medium 1 on the side of the photo-conductive layer 4 thereof through a specific gap. The recording medium 5 is composed of a substrate 8, an electrode 7 and a charge holding layer 6 laminated in order. The charge holding layer 6 thus faces the photo-conductive layer 4 in the configuration.
A d.c. voltage source 9 applies a voltage across the electrodes 3 and 7 through a switch 10.
In the configuration, when the electro-magnetic wave is emitted to the glass substrate 2 to pass therethrough and reach the photo-conductive layer 4 through the electrode 3, the impedance thereof is varied accordingly with the intensity of the electro-magnetic wave. A charge latent image according to the intensity is thus recorded on the charge holding layer 6 of the recording medium 5 by discharge of the electric field generated due to the voltage which is applied across the electrodes 3 and 7.
This configuration has the disadvantage that the photosensitive medium 1 or recording medium 5 may be broken down when the voltage is rapidly applied thereto.
The cause of the breakdown will be explained with reference to FIG. 2 showing the graphs which represent voltages of the parts in the configuration in the case of applying the voltage across the electrodes 3 and 7.
In FIG. 2, the graph A expresses the inter-electrode voltage applied across the electrodes 3 and 7, the graph B the gap voltage across the gap and the graph C the surface potential of the recording medium 5 due to discharged charges, and the point P the voltage across the gap when the inter-electrode voltage is initially applied.
The switch 10 is closed to cause the d.c. voltage source 9 to apply the voltage across the electrodes 3 and 7. The inter-electrode voltage rises momentarily and rapidly at the time, as represented by the graph A. The gap voltage (the graph B) at the time is depicted by the point P which is the voltage of the gap divided by each equivalent capacity of the photo-conductive layer 4, the gap and the charge holding layer 6 and exceeds over the discharge-starting voltage V.sub.B.
Discharge then occurs and the surface potential of the medium 5 (the graph C) rises with a certain time constant until the gap voltage becomes lower than the discharge-starting voltage V.sub.B.
As is already described, the momentary large voltage to the gap and also the momentary rise of discharge voltage may cause the breakdown of the solid portion of the photo-conductive layer 4 and the charge holding layer 6. Irregularity or pin holes on the solid portion which lead the electric field to converge thereon also may cause breakdown.
Next, a charge latent image recording/reproducing system using photo-to-charge and photo-to-photo transducers will be explained with reference to FIGS. 3 and 4.
In FIG. 3, a d.c. voltage source 11 is connected across the electrode 3 of the photosensitive medium 1 and the electrode 7 of the recording medium 5 to apply a voltage V.sub.1 thereacross. The other parts of the configuration and the fundamental charge latent image recording operation are same as those shown in FIG. 1.
While in FIG. 4, a charge latent image reading member 12 is composed of an electrode 13 allowing a reading light to pass therethrough, a photo-modulation layer 14 modulating the reading light accordingly with the intensity of charges and a dielectric mirror layer 15 laminated in order. The reading light is a modulative electro-magnetic wave including an electro-magnetic radiation beam such as X-rays, .gamma.-rays, radio wave or light.
The recording medium 5 which is storing the charge latent image is placed to face the charge latent image reading member 12 on the side of the dielectric mirror layer 15 thereof such that the charge holding layer 6 faces the dielectric mirror layer 15. Furthermore, the electrode 7 of the recording medium 5 and the electrode 13 of the reading member 13 are connected to each other.
In the configuration, when a reading light is emitted to the electrode 13 through a semi-transparent mirror 16 to reach the photo-modulation layer 14 through the electrode 13, the electric field corresponding to the charge latent image distribution which has been recorded on the charge holding layer 6 is applied to the photo-modulation layer 14, which modulates the reading light accordingly with the charge latent image distribution.
The modulated light is reflected in the dielectric mirror 15 and passes the electrode 13. The direction of the modulated light is then changed by the semitransparent mirror 16. The modulated light is thus read out.
There are disadvantages in the configuration that the discharge is generated across the reading member 12 and the recording medium 5 when the charge latent image is read out from the recording medium 5. This causes the charge latent image to be damaged or the reading member 12 to be broken down.